Fertilizers have been used for many years to supplement plant nutrients in soil or other growing media. In recent years the art has focused on techniques to deliver controlled amounts of plant nutrients to the soil or other growing media. It is recognized, for example, that controlling the release of plant nutrients such as nitrogen from highly soluble fertilizer granules is desirable because releasing the nutrients over an extended period of time achieves advantages which include increased efficiency of fertilizer use by plants, reduced application costs since fewer applications of fertilizer are required and reduced nutrient loss caused by leaching and denitrification. Applying a coating on the surface of the fertilizer granules may reduce the dissolution rate of the granules and impart controlled-release characteristics. In essence, the water in the soil and rainwater are kept away from the very soluble fertilizer until a granule develops a flaw such as a crack or fissure in the coating or the coating develops porosity upon exposure to water.
In some applications, a sulfur coating is applied to fertilizer granules. Due to the inherent brittleness of the crystalline solid sulfur-coating and the thin, or even non-continuous, coating on many of the granules, it is typical to apply some type of secondary outer coating or sealant onto the sulfur-coated surface. This sealant conventionally is either a polymeric hydrocarbon, petroleum-based wax, or a combination of high viscosity polymeric paraffinic oil plus polyethylene, which is spray-applied as a hot melt liquid onto the hot, but solidified sulfur-coating surface.
Although these sulfur coated fertilizers have received substantial uses, there are problems from the standpoint of obtaining uniform coating thicknesses, obtaining predictable release characteristics, abrasion and impact resistance, and the complexity of the coating process steps. Because of these problems associated with sulfur coated fertilizers, polymer coated fertilizers have received substantial attention, particularly in view of the improved controlled release properties obtained with certain polymer coated fertilizers at lower coat weights. The polymer coated fertilizers may have multiple coating layers. Examples of polymeric fertilizer coatings include: an inner coating of a urethane reaction product derived from reacting isocyanate and polyol, with an outer coating of an organic wax; an oleo polyol(s) coating; a polyurea coating formed by applying an isocyanate-reactive component containing at least two amine groups and subsequently applying a polyisocyanate.
Various polymeric yet sulfur containing coatings are also known. Examples of such coatings include: sulfur containing isocyanate coatings prepared by applying a mixture of sulfur and an isocyanate to the fertilizer and then applying an isocyanate-reactive material; a coating formed by applying a sulfur coating and subsequently applying a polymer coating over the sulfur; a thermosetting resin coating such as urethane and a hydrophobic compound (e.g., wax); a multilayer coating comprising a polyurethane layer and a wax layer; and a multilayer coating comprising a polymer layer, a sulfur layer, and another polymer layer.
Other coatings are also known. For example, a fertilizer core can be coated with organic oil and particles to improve the release profiles of the fertilizer.